The present invention relates to a magnetoresistive element having ferromagnetic double tunnel junction, and, a magnetic memory device using the same.
The magnetoresistance effect is a phenomenon that electrical resistance changes when a magnetic field is applied to a ferromagnetic material. As the magnetoresistive element (MR element) using the above effect has superior temperature stability within a wide temperature range, it has been used for a magnetic head and a magnetic sensor, and the like. Recently, a magnetic memory device (a magnetoresistive memory or a magnetic random access memory (MRAM)) has also been fabricated. The magnetoresistive element has been required to have high sensitivity to external magnetic field and quick response.
In recent years, there has been found a magnetoresistive element having a sandwich film in which a dielectric layer is inserted between two ferromagnetic layers, and uses tunnel currents flowing perpendicularly to the film, so-called a ferromagnetic tunnel junction element (tunnel junction magnetoresistive element, TMR). The ferromagnetic tunnel junction element shows 20% or more of a change rate in magnetoresistance (J. Appl. Phys. 79, 4724 (1996)). Therefore, there has been an increased possibility to apply the TMR to a magnetic head and a magnetoresistive memory. However, there is a problem that the magnetoresistance (MR) change is considerably decreased in the ferromagnetic single tunnel junction element, when a voltage-to be applied is increased to obtain required output voltage (Phys. Rev. Lett. 74, 3273 (1995)).
There has been proposed a ferromagnetic single tunnel junction element having a structure in which an antiferromagnetic layer is provided in contact with one ferromagnetic layer for the ferromagnetic single tunnel junction to make the ferromagnetic layer to be a magnetization pinned layer (Jpn. Pat. Appln. KOKAI Publication No. 10-4227). However, such an element also has a similar problem that the MR change is considerably decreased when an applied voltage is increased to obtain required output voltage.
On the other hand, there has been theoretically estimated that a magnetoresistive element having a ferromagnetic double tunnel junction forming a stacked structure of Fe/Ge/Fe/Ge/Fe has an increased MR change owing to spin-polarized resonant tunnel effect (Phys. Rev. B56, 5484 (1997)). However, the estimation is based on results at a low temperature (8K), and therefore the above phenomenon is not necessarily caused at room temperature. Note that the above element does not use a dielectric such as Al2O3, SiO2, and AlN. Moreover, as the ferromagnetic double tunnel junction element of the above structure has no ferromagnetic layer pinned with an antiferromagnetic layer, there is a problem that the output is gradually decreased owing to rotation of a part of magnetic moments in a magnetization pinned layer by performing writing several times when it is used for MRAM and the like.
In addition, there has been proposed a ferromagnetic multiple tunnel junction element comprising a dielectric layer in which magnetic particles are dispersed (Phys. Rev. B56 (10), R5747 (1997); Journal of Applied Magnetics, 23, 4-2, (1999); and Appl. Phys. LeTT. 73 (19), 2829 (1998)). It has been expected that the element may be applied to a magnetic head or a magnetoresistive memory, as 20% or more of an MR change has been realized. In particular, the ferromagnetic double tunnel junction element has an advantage that the reduction in the MR change can be made low even with increased applied voltage. However, as the element has no ferromagnetic layer pinned with an antiferromagnetic layer, there is a problem that the output is gradually decreased owing to rotation of a part of magnetic moments in a magnetization pinned layer by performing writing several times when it is used for MRAM and the like. As a ferromagnetic double tunnel junction element using a ferromagnetic layer consisting of a continuous film (Appl. Phys. Lett. 73(19), 2829 (1998)) has a ferromagnetic layer consisting of a single layer film of, for example, Co, Ni80Fe20 between dielectric layers, there are problems that a reversal magnetic field for reversing the magnetic moment may not be freely designed, and that coercive force of the ferromagnetic layer may be increased when the material such as Co is processed.
For application of the ferromagnetic tunnel junction element to MRAM and the like, external magnetic fields are applied to a ferromagnetic layer (free layer, or a magnetic recording layer), magnetization of which is not pinned, by flowing current in a wire (bit line or word line) in order to reverse the magnetization of the magnetic recording layer. However, since increased magnetic fields (switching magnetic fields) are required for reversing the magnetization of the magnetic recording layer as memory cells become smaller, it is necessary to flow a high current in the wire for writing. Thus, power consumption is increased for writing as memory capacity of the MRAM is increased. For example, in an MRAM device with a high density of 1 Gb or more, there may be caused a problem that the wires melt owing to increased current density for writing in the wires.
As one solution for the above problem, an attempt is made to carry out magnetization reversal by injecting spin-polarized current (J. Mag. Mag. Mat., 159 (1996) L1; and J. Mag. Mag. Mat., 202(1999) 157). However, the method for performing magnetization reversal by injection of the spin current causes increase in current density in the TMR element, which leads to destruction of a tunnel insulator. Moreover, there have been no proposals for an element structure suitable for spin injection.